GB2161809A - Carboxylic acid esterification - Google Patents

Abstract

A process for the esterification of carboxylic acids such as free fatty acids present in palm oil or palm oil fractions comprises refluxing a mixture of the carboxylic acid with an alcohol in the presence of solid ammonium bisulphate which is then removable by filtration. The process has particular application to the esterification of the free fatty acid content of a mixture of free fatty acids and glycerides followed by transesterification of the glycerides using an alkaline catalyst.

Description

SPECIFICATION Carboxylic acid esterification The present invention relates to a process for esterification of carboxylic acids employing solid ammonium hydrogen sulphate as a catalyst.

It is well known that carboxylic acids can be esterified by reaction with an alcohol in the presence of a mineral acid catalyst such as sulphuric acid or hydrochloric acid.

Separation of the mineral acid is necessary when the esterification reaction is complete.

Separation of liquid mineral acid from the reaction mixture may not be a straightforward procedure.

The invention has particular applicability to the esterification of free fatty acids in admixture with triglycerides.

It is known, see for instance British Patent Specification Nos. 2051786 and 2072167, to transesterify the triglyceride components of such mixtures by reaction with an excess of lower alcohol in the presence of a basic catalyst such as sodium hydroxide after having first esterified the free fatty acid content of the mixture.

The esterification serves to prevent the free fatty acids forming soaps with the base added as the transesterification catalyst.

When liquid acids are used as the esterification catalyst however, they present a separation problem.

The invention has particular applicability to the esterification and subsequent transesterification of acid rich fractions derived from vegetable oil milling and refining procedures, e.g. the refining of palm oil. In the physical refining of palm oil, free fatty acids are distilled off under vacuum e.g. at 260 to 270or, so producing a fraction known as palm fatty acid distillate (PFAD). This fraction contains typically 80 to 90% free fatty acid, the bulk of the remainder being glycerides. A second acid rich palm oil product or fraction obtained from palm oil mills is sludge oil which typically contains 70 to 80% free fatty acid. These fractions have in the past been regarded essentially as waste products, but by the esterification process of the present invention can be converted into valuable esters.After separation of the solid catalyst employed in the present invention, the glyceride component of the mixture may be transesterified under basic catalysis.

The present invention provides a process for the esterification of a carboxylic acid which comprises reacting a carboxylic acid or a mixture of such acids with one or more alcohols in the presence of solid ammonium bisulphate as a catalyst.

Preferably, the reaction is carried out with heating, e.g. to refluxthe alcohol.

It has been found that the major part of the bisulphate remains solid through the reaction and can be separated by filtration.

The process is particularly applicable to the treatment of long chain carboxylic acids such as occur naturally in vegetable oils. Preferably therefore the carboxylic acid is a C10 to C25 carboxylic acid or a mixture of such acids, more preferably a C15 to C18 acid or mixture thereof.

The carboxylic acids may be saturated or unsaturated e.g. with one or two double bonds.

Generally speaking, it is preferred that the esterification reaction be carried out on a naturally occurring mixture of carboxylic acids such as those derived from vegetable oil or on a fraction derived from such a mixture. For instance, the acids may be derived from palm oil either in its crude state or after some treatment by way of refining. The acids may be contained in a palm oil fraction.

The present invention is of particular application when substantial quantities of glycerides are present together with the fatty acids subjected to the esterification. The process has especial applicability to naturally occurring mixtures of free fatty acids and glycerides and to fractions obtained therefrom.

The process has particularly applicability to fatty acid distillates obtained from vegetable oils such as palm oil and to sludge oils derived from the refining and milling of such vegetable oils. As explained above these fractions are very rich in fatty acids, for instance containing above 70% by weight free fatty acid. The remainder ofthesefractions is chiefly made up bytriglycerides but these fractions also contain some minor inredients of considerable intrinsic value such as sterols, to tocotrienols, carotenes and tocopherols.

Preferably, the esterification of the acid is with one or more Ci to C8 more preferably C1 to C2 alcohols, especially methanol. Esters of vegetable oils with such lower alcohols are known to be useful as fuels for diesel engines either alone or in admixture with conventional diesel mineral oil fuel.

Preferably, there is employed from 0.5% to 30% by weight of the bisulphate based upon the weight of carboxylic acid, more preferably 1 to 20% by weight e.g. about 10% by weight. Suitably, in a commercial scale esterification, the bisulphate can be used in pelletised form and can be carried out continuously.

Alkyl esters of carboxylic acids, e.g. the lower alkyl esters described above, can be obtained from glyceride mixtures containing free fatty acid both by the esterification of the free fatty acid and by transesterification of the glyceride component.

In a preferred process according to the invention, the free carboxylic acid component of a mixture of glycerides and free carboxylic acids is esterified by a process as described above and the glycerides are then subjected to a transesterification reaction with the ester produced in the first stage preferably left in situ.

The transesterification reaction is conveniently carried out using a basic catalyst such as sodium hydroxide. Methods of this general kind fortransesterifying glycerides are described in British Patent Specifications 2051786 and 2072167.

In particular, the transesterification step may be carried out following removal of the bisulphate catalyst, e.g. by filtration, by adding to the glyceride/ester mixture a catalyst such as an alkali metal hydroxide together with alcohol if not aiready present in sufficient quantity and preferably heating, e.g. to reflux the alcohol. The alkali catalyst level is preferably from 0.05 to 2.0% by weight of the glyceride/free fatty acid mixture originally employed, more preferably from 0.1 to 1.0% e.g. 0.3 to 0.7% by weight. Sodium hydroxide is the preferred catalyst The process is conveniently carried out using excess alcohol as a solvent, e.g. using a molar excess of methanol over glyceride of more than 6:1, more preferably more than 8:1, e.g. more than 15:1.

The transesterification reaction is generally essentially quantitative and the choice of catalyst level and temperature is largely governed by the desired reaction time.

Apart from the value of the lower alkyl esters of fatty acids obtainable by the above combination of an esterification and a transesterification reaction of fractions of vegetable oil, transforming the glycerides and the free fatty acids into lower alkyl esters provides a convenient method for enabling the ready separation of these components, e.g. by vacuum distillation, from the minor components of high intrinsic value such as tocopherols mentioned above.

Accordingly, the present invention includes a method of separating such intrinsically valuable minor constitutents of a vegetable oil or vegetable oil fraction which comprises esterifying free fatty acids in the vegetable oil orvegetable oil fraction,transesterifying glyceride components ofthevegetable oil or vegetable oil fraction and separating the fatty acid esters produced from the said minor components, e.g. by vacuum distillation, wherein the esterification reaction is carried out using solid ammonium bisulphate as a catalyst.

The invention includes sterols, tocopherols, tocotrienols and carotenes separated from vegetable oils or vegetable oil fractions in this way.

The invention will be illustrated by the following Examples provided for the sake of illustration.

Example 1 Crude palm oil (309) was weighed into a 2-necked flask and preheated to about 60"C. methanol (15ml) was then added and the mixture reheated to about 60"C. Ammonium bisulphate (0.9) was added and the mixture heated at reflux (64"C) for a specified time (see Table V below). The mixture was stirred at a constant speed throughout the reaction. At the end of the specified time, the reaction mixture was transferred to a separating funnel and immediately washed until the washing was neutral. The washed product was pumped dry using a vacuum pump and the fatty acid content checked by titration. The procedure was repeated using crude palm stearin (309). The results are set forth in Table I below in which CPO means crude palm oil and CPS means crude palm stearin.

TABLE I Reaction Time FFA % Esterification mins initial final (Run 1) (CPO) 10 4.23 2.57 39.2 30 4.22 1.37 67.5 60 4.25 0.77 81.9 120 4.26 0.27 93.7 (Run 2) (CPS) 10 4.70 2.93 37.7 30 4.74 1.61 66.0 60 4.74 0.92 80.6 120 4.73 0.32 93.2 Example 2 The procedure of Example 1 was repeated except that the reaction time was fixed at 1 hour but the temperature varied as set forth in Table II below, which table also sets forth the results obtained. In Table VI CPO is crude palm oil and CPS is crude palm stearin.

TABLE II Temperature FFA % Esterification "C initial final (Run 1) 64 4.25 0.77 81.9 (CPO) 53 4.26 1.27 70.2 40 4.26 2.11 50.5 29 4.23 2.32 45.2 (Run 2) (CPO) 64 9.85 2.10 78.7 50 9.85 3.90 60.4 40 9.84 6.89 30.0 29 9.84 5.57 43.4 (Run 3) (CPS) 64 4.7 1.25 73 50 4.7 1.75 63 40 4.7 1.86 60 29 4.7 3.53 25 Example 3 The procedure of Example 1 was repeated except that the amount of ammonium bisulphate used was 0.39. The results are set forth in Table Ill below.

TABLE Ill Reaction Time FFA % Esterification mins initial final 10 9.79 8.39 14.3 60 9.87 5.97 39.5 120 9.83 4.27 56.6 Example 4 The procedure of Example 1 was repeated exceptthat 17 ml of methanol was used, the reaction time was fixed at 1 hour and the amount of catalyst was varied as set forth in Table IV below, which table also sets forth the results obtained.

TABLE IV Catalyst (g) FFA % Esterification initial final initial final 0.1003 0.1339 9.83 8.89 9.6 0.3028 0.4197 9.87 6.07 38.3 0.9018 0.8092 9.83 2.10 78.6 1.5045 1.4052 9.79 0.91 90.7 Example 5 The procedure of Example 2 was repeated except that 0.39 amonium bisulphate was used. The results are setforth in Table V below.

TABLE V Reaction Temp- FFA % Esterification erature - OC initial final 64 9.83 5.05 48.6 50 9.85 7.38 25.1 40 9.84 9.02 8.3 29 9.83 7.85 20.1 Example 6 Crude palm oil (309) having a free fatty acid content of 4.3% was weighed into a 2-necked flask. The oil was preheated to 60"C, methanol (1 Oml) added and the mixture reheated to 600C. Ammonium bisulphate (0.99) was added and the mixture heated at reflux (64 C) for 1 hour. Sodium hydroxide (1.59) in methanol (1 Oml) was then added to the reaction mixture and heating continued for a further 30 mins. The reaction was found to be complete (100% conversion of triglycerides to methyl esters).Two layers were observed, methyl esters on top and glycerol at the bottom with some solid settled at the bottom of the flask.

Example 7 15g of tallow was weighed into a pecked flask and preheated to about 60"C. Methanol (7.5ml) was then added and the mixture reheated to about 60"C. Ammonium bisulphate (0.39) was added and the mixture heated at reflux (64"C) for 1 hour. The mixture was stirred at a constant speed throughout the reaction. At the end of 1 hour, the reaction mixture was transferred to a separating funnel and immediately washed with distilled water until the washing was neutral. The washed product was pumped dry using a vacuum pump and the fatty acid content checked by titration.

The above procedure was repeated using soyabean oil, sunflower oil, corn oil, cocunut oil, palm kernel oil and rapeseed oil.

In the case of corn oil, palm kernel oil, and rapeseed oil, 1 2g of the respective oil, 6ml methanol, and 0.12g ammonium bisulphate were used. Otherwise, the reaction conditions were the same as above.

The results are set forth in Table Vl below.

TABLE VI FFA % Esterification Oil Initial Final Tallow 3.9 2.8 28.2% Soyabean Oil 1.3 1.2 7.6% Sunflower Oil 3.5 3.1 11.4% Corn Oil 9.6 6.5 32.3% Coconut Oil 1.5 0.5 66.7% Palm Kernal Oil 6.3 3.1 50.8% Rapeseed Oil 3.2 2.8 12.5% Example 8 The procedure of Example 2 was repeated using 0.3g ammonium bisulphate and 15ml methanol, absolute ethanol, isopropyl alcohol or n-butanoi respectively. The results are set forth in Table VII below. In the case of isopropyl alcohol and n-butanol, esterification did not seem to have taken place.

TABLE VII FFA % Esterification Alcohol Initial Final Methanol 9.87 5.97 39.5 Ethanol (absolute) 10.1 6.8 32.7 Isopropyl alcohol 10.099 10.035 0.6 n-butanol 10.1 10.1 0 Note: In the case of (3) and (4) above, probably no esterification has taken place.

Example 9 Palmitic acid (159) and oleic acid (159) was weighed in a 2-necked flask and preheated to 60"C. Methanol (15 ml) was then added and the mixture heated to about 60"C. Ammonium bisulphate (0.3 g) was added and the mixture heated at reflux (64"C) for 2 hours. The mixture was stirred at a constant speed throughout the reaction. At the end of 2 hours, the reaction mixture was transferred to a separating funnel and immediately washed with distilled water until the washing was neutral. The washed product was pumped dry using a vacuum pump and the fatty acid content checked by titration. It was found that 58.1% of the acid mixture had been esterified.

Claims (19)

1. A process for the esterification of a carboxylic acid which comprises reacting a carboxylic acid or a mixture of such acids with one or more alcohols in the presence of solid ammonium bisulphate as a catalyst.

2. A process as claimed in Claim 1, wherein the carboxylic acid is a C10 to C25 carboxylic acid or the said mixture is a mixture of such acids.

3. A process as claimed in Claim 1 or Claim 2, wherein the esterification is carried out in the presence of one or more glycerides.

4. A process as claimed in any one of the preceding claims, wherein a naturally occurring mixture of carboxylic acids or a fraction derived from such a mixture is subjected to the esterification.

5. A process as claimed in Claim 4, wherein acids contained in palm oil, a treated palm oil or a palm oil fraction are subjected to the esterification.

6. A process as claimed in Claim 4 or Claim 5, wherein a fatty acid distillate obtained from a vegetable oil or a sludge oil derived from a vegetable oil is subjected to the esterification.

7. A process as claimed in any one of the preceding claims, wherein the acid or acid mixture is esterified with one or more C1 to C2 alcohols.

8. A process as claimed in Claim 7, wherein the acid or acid mixture is esterified with methanol.

9. A process as claimed in any one of the preceding claims, wherein there is employed from 0.5% to 30% by weight of the bisulphate based upon the weight of carboxylic acid.

10. A process as claimed in Claim 9, wherein there is employed from 1 to 20% by weight of catalyst based upon the weight of carboxylic acid.

11. A process as claimed in Claim 1 and substantially as hereinbefore described in any one of the Examples.

12. A carboxylic acid ester or mixture thereof produced by a process as claimed in any one of Claims 1 to 11.

13. A process for the transesterification of g lycerides containing free carboxylic acids, which process comprises first esterifying the carboxylic acids by a method as claimed in any one of the preceding claims and then subjecting the glycerides to a transesterification reaction.

14. A process as claimed in Claim 13, wherein following the esterification, the bisulphate catalyst is separated from the reaction mixture and a transesterification catalyst is added thereto and the mixture is heated in the presence of an alcohol to carry out said transesterification reaction.

15. A process as claimed in Claim 13 or Claim 14,wherein the transesterfication reaction is carried out using an alkaline transesterfication catalyst.

16. A process as claimed in Claim 15, wherein the transesterification catalyst is sodium hydroxide.

17. A process for the esterification and transesterification of a fatty acid/glyceride substantially as hereinbefore described in any one of the Examples.

18. A carboxylic acid ester produced by a process as claimed in any one of Claims 13 to 17.

19. A liquid fuel comprising one or more esters as claimed in Claim 11 or Claim 18 in admixture with other liquid fuel constituents.